Definitely put some water absorber in the gas tank. If the tank isn't full put new high octane high quality gas in it ASAP. If it's full you may want to add a fuel stabilizer/fuel system cleaner/octane enhancer. It's possible that the light fraction in the gas which helps it start have evaporated.
Check tires carefully, sitting uncovered it's possible the fabric has absorbed water and that can cause separation at highway speed. If you decide to drive on them take it easy and stop and check frequently. I would probably try to drive on them myself and see if they are ok. I've had several tires delaminate on me and aside from an adrenaline rush and some sweating on the side of the road I've never felt there was much danger. I've always been able to sort of feel it coming.
Battery - of course it's discharged and you may be able to recharge it, but maybe not. Take a charger and cables, or a new battery.
When you change the oil look at the oil you drain out for anything suspicious.
18 months isn't that long and it will probably be OK. I cousin once bought an MGA that had been sitting in a barn for several years. He change the oil, battery and gas then drove it home.